High frequency (“HF”) generators are used to generate HF power and to deliver it to a load including, for example, plasma processes, such as plasma coating and plasma etching. Since the impedance of the load may change, and as a consequence, in the event of mismatching, (partial) reflection of the power delivered by the HF generator may occur, the total power supplied by the HF generator is often not absorbed in the load (the plasma). In order to be able to adjust accurately or regulate the HF power delivered to the load, it is desirable to determine the power absorbed in the load.
A directional coupler can be used to measure/determine the HF power absorbed in a load based on the difference between the power generated by the HF generator and the reflected power. It is therefore possible to control the HF generator in such a way that the power absorbed in the load is very accurately adjusted and can be maintained at a constant level.
The measurement by a directional coupler or by a reflectometer of the power delivered to a load (incident power, forward power), and the measurement of the reflected power is nevertheless prone to error due to the finite directivity of the directional coupler. The measurement of the power absorbed in the load is therefore likewise prone to error. A phase measurement between the incident and the returning wave (power delivered to the load and the power reflected by the load) can be used to compensate for this error.
Another possible method of accurately measuring the power delivered into the load is to measure the current and the voltage. However, in this case, a very good decoupling of the voltage sensor and the current sensor must be achieved. In addition, a very accurate phase measurement between the voltage and the current is required.
FIG. 1a shows a high frequency system 11 having a high frequency generator 100 as source, a coupler arrangement 12 with a coupler 200 in the form of a directional coupler, plus a load 300 as drain. The coupler has a main line 290 having a first and second port 1, 3 and a coupling line 291 with coupling line ports 2, 4, which are terminated by impedances 205, 206, which correspond to the nominal impedance Z0 of the high frequency system 11 or at least to that of the coupler 200. At the port 2, a proportion of the incident wave at the corresponding port 1 of the main line 290 can be observed with a measuring device 207; correspondingly, at the port 4 a proportion of the wave reflected by the load and incident at the corresponding port 3 of the main line can be observed with a measuring device 208. The magnitude of the proportion depends in each case on the coupling factor. In an ideal coupler, the directivity is infinitely large, and at the port 2 no proportion of the wave incident at port 3 will be observed and at port 4 no proportion of the wave incident at port 1 will be observed.
FIG. 1b likewise shows a high frequency system 10. The coupler 240 comprises coupling lines 291, 292. At the port 2 of the first coupling line 291, a proportion of the wave fed in at the port 1 of the main line 290 is measured, at port 4 of the second coupling line 292 a proportion of the wave fed in at port 3 of the main line 290 is measured. The ports 223, 4 of the second coupling line 292 are terminated by the impedances 225, 226, which correspond to the nominal impedance Z0.
To describe n-port networks, such as a directional coupler for example, it is customary to consider wave amplitudes or wave strengths at the ports instead of the voltage sources. In this connection, the wave strength of an incident wave at port i is normally denoted by the letter ai and the wave strength of the outbound wave at port i is denoted by the letter bi. Furthermore, the outbound and incident waves are typically linked by way of a scattering matrix.
A directional coupler can be regarded as a four-port network, the transmission characteristic of which can be described by the complex variables of coupling factor and isolation. In FIG. 1b, the directional coupler 10 is illustrated as a four-port network with coupling lines 291, 292 that are arranged on both sides of the main line 290. Here, Cf=Cf·ejφCf is the coupling factor in the forward direction, i.e. from port 1 to port 2. The coupling factor in the reverse direction is Cr=Cr·ejφCr, i.e. from port 3 to port 4. Correspondingly, If=If·ejφIf is the isolation in the forward direction (port 1 to port 4) and Ir=Ir·ejφIr is the isolation in the reverse direction (port 3 to port 2). The variables Cf, Cr, If, Ir are in the present case the reciprocal values of the scattering parameters of the scattering matrix. Provided that 1<<λ:
                    a        3            _        ≈                  b        1            _                          b        1            _        =                  Γ        _            ·                        a          1                _                                b        4            _        =                                                      a              1                        _                    ·                      1                                          I                f                            _                                      +                                            a              3                        _                    ·                      1                                          C                r                            _                                          ≈                                    a            1                    _                (                              1                                          I                f                            _                                +                                    Γ              _                                                      C                r                            _                                      )            
Γ being the reflection factor.